Smear black mass (4)
I believe the answer is:
blot
'smear' is the definition.
(I know that smear can be written as blot)
'black mass' is the wordplay.
'black' becomes 'b' (abbreviation as in pencil classifications).
'mass' becomes 'lot' (synonyms).
'b'+'lot'='BLOT'
(Other definitions for blot that I've seen before include "Dark mark or stain" , "bad mark" , "Stain (on landscape?)" , "Ink mark" , "Blemish made with ink" .)